In the papermaking art, an aqueous suspension containing cellulosic fibers, and optional fillers and additives, is fed into a headbox which ejects the suspension onto a forming wire. Water is drained from the suspension through the forming wire so that a wet web of paper is formed on the wire, and the paper web is further dewatered and dried in the drying section of the paper machine. Drainage and retention aids are conventionally introduced into the suspension in order to facilitate drainage and increase adsorption of fine particles onto the cellulosic fibers so that they are retained with the fibers on the wire.
Aqueous suspensions containing cellulosic fibers may contain, apart from cellulosic fibers, also compounds which have negative impact on the production process. Such compounds can be found both in suspensions originating from virgin pulp and from recycled pulp. Compounds which are released during the pulping and bleaching operations are commonly referred to as pitch. Examples of pitch include wood resins such as lipophilic extractives (fatty and resin acids, sterols, stearyl esters, triglycerides) and also fats, terpenes, terpeniods, waxes etc. These compounds contribute to a high anionic charge of the suspensions.
Further, especially in closed mills where white water is extensively re-circulated, the suspensions may also comprise charged contaminants like salts and various wood polymers of which the charged, low charged and non-charged compounds compete with the cellulose with respect to the adsorption of and interaction with added performance chemicals such as drainage and retention aids, sizing agents, etc. Usually such disturbing compounds are referred to as anionic trash.
All the above-mentioned compounds interfere with the pulp and paper making processes in various ways. To compensate for the high anionic charge caused by disturbing substances, increased amounts of cationic additives, which contribute to improved dewatering and retention, have been used in the papermaking processes.
U.S. Pat. No. 4,388,150 discloses the use of a cationic starch together with colloidal silicic acid to improve retention and drainage and improve characteristics of resulting paper in a papermaking process in which mineral fillers are used.
EP-A 0 700 473 discloses a papermaking process in which retention and/or dewatering are improved by adding a cationic long-chain polyacrylamide to an aqueous suspension containing cellulosic fibers and then a polymeric aluminum salt and a base or an acid.
Despite the fact that considerable improvements have been achieved in the drainage and retention of the aqueous suspensions containing cellulosic fibers, there is still a need for improvements, especially when producing cellulosic products from aqueous cellulosic suspensions derived from mechanical pulps. Therefore, it is an object of this invention to provide a process for the production of a cellulosic product with further improvements in drainage and retention of the production process. Further objects will appear hereinafter.